deck_11268041 Flashcards by 麦兜

what is a MAGNETIC FIELD?

where a magnet/magnetic material experiences force

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examples of MAGNETIC MATERIAL

iron, steel, nickel, cobalt

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what happens to magnetic materials when they’re next to magnets? why?

ATTRACTED because of INDUCED MAGNETISM (domains line up)

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how do you DESTROY A MAGNET (remove magnetism)?

make the domains point in different directions:- hammer it- heat it- wrap a coil and pass alternating current

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why doesn’t a normal piece of magnetic material (e.g. iron) act like a magnet?

domains point at different direction and cancel out

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how do you MAKE A MAGENT from magnetic material?

make the domains line up:- stroking- put next to another magnet- wrap coil + pass current

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difference between IRON and STEEL’S MAGNETISM

IRON: soft - easy to magnetize and demagnetize (immediately loses magnetism when magnetic field is removed)STEEL: hard - hard to magnetize and demagnetize (retains magnetism even after magnetic field is removed)

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property of INDUCED MAGNETS

TEMPORARY MAGNETISM, loses magnetism when magnetic field is removed

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how do you SHOW THE DIRECTION of MF?

small compasses in a circle → needles point in the direction of field lines

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magnetic field lines always go from … to … outside the magnet

N to S

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DENSER magnetic field lines means…

STRONGER magnetic field

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uses of HARD magnetic materials

making permanent magnets, loudspeaker

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uses of SOFT magnetic materials

motors, generators, electric bells, transformer cores

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differences between ELECTROMAGNETS and PERMANENT MAGNET

PERMANENT MAGNET:- constant strength of magnetic field- can’t be switched on/off- N and S can’t be changedELECTROMAGNET:- variable strength of magnetic field- can be quickly switched on/off- N and S can be changed by changing direction of current

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how do you measure the strength of an electromagnet?

measure the mass of paper clips it can hold

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do you use iron or steel as the core in the LOUDSPEAKER? why?

STEEL cuz its HARD magnetic material - doesn’t lose magnetism easily

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do you use iron or steel as the core in the RELAY? why?

IRON cuz its SOFT magnetic material - can change magnetism easily

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what happens when electric current flows through a wire?

MAGNETIC FIELD is created

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difference between A.C. and D.C.

D.C. → doesn’t change direction → constant MF| A.C. → changes direction → alternating MF

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how does the pattern of magnetic field look like outside a SOLENOID?

same as bar magnets

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how does the pattern of magnetic field look like outside a CURRENT-CARRYING WIRE?

circular, perpendicular to wire

how do you make an electromagnet stronger?

• increase current/voltage• increase number of turns in coil→ increased strength of MF

how do you determine the DIRECTION OF MF in a CURRENT-CARRYING WIRE?

right hand rule:• thumb - direction of current• fingers - direction of MF

how do you determine the DIRECTION OF MF in a SOLENOID?

right hand rule:• thumb - points to N pole• fingers - direction of MF

explain the MOTOR EFFECT

power supply gives VOLTAGE across coil → CURRENT flows through coil → MF induced around coil → INTERACTS with permanent magnet's MF → interaction creates FORCE → coil spins

MOTOR EFFECT:| how to increase the size of force?

* increase current/voltage* increase turns of coil* stronger magnet → stronger MF

MOTOR EFFECT:| how to reverse the direction of force?

* reverse direction of current (battery)| * reverse direction of MF

uses of motor effect

hairdryer, blender, electric drill, etc.

MOTOR EFFECT:| how to determine direction of force/current/MF if others are known?

right-hand slap rule:拇指 - Current 其他手指 - Magnetic Field手心 - Force fleming's left-hand rule:拇指 - F (force) 食指 - B (magnetic field) 中指 - I (current)

energy transfer in D.C. MOTOR

electrical → kinetic

D.C. MOTOR:| what component is needed? why?

SPLIT-RING COMMUTATOR| ensures current always flow the same way → continuous rotation of coil

explain the GENERATOR EFFECT (also known as electromagnetic induction)

electric conductor experiences CHANGE IN MF → VOLTAGE is induced → CURRENT is generated

GENERATOR EFFECT:| explain 2 ways in which you can induce current + observation to show current is induced

move a wire (connected to galvanometer) up and down between N and S poles of a magnet → cuts through MF lines → induces V+I → galvanometer needle movesmove a magnet in and out of a coil (connected to galvanometer) → cuts through MF lines → induces V+I → galvanometer needle moves

GENERATOR EFFECT:| what device do you use to show current is induced?

galvanometer

GENERATOR EFFECT:| how to increase the size of induced current?

* faster motion* more turns of coil* stronger magnet → stronger MF

A.C. GENERATOR:| why is an alternating current (a.c.) produced instead of direct current (d.c.) ?

one side of the coil moves up → induces VOLTAGE in one direction → induces CURRENT in one direction... rotation continues ... same side of the coil moves down → induced VOLTAGE reverses direction → induced CURRENT reverses direction*this repeats and produces a constantly changing current

A.C. GENERATOR:| when is the induced V+I at maximum?

when coil moves PERPENDICULAR TO MF → cutting MF

A.C. GENERATOR:| when is there no induced V+I at all?

when coil moves PARALLEL TO MF → not cutting MF

A.C. GENERATOR:| what components are needed? why?

SLIP RING + BRUSH| maintain contact between coil and external circuit

energy transfer in A.C. GENERATOR

kinetic → electrical

uses of generator effect

wind turbines

what is the function of a TRANSFORMER?

change V to a bigger/smaller value

STEP-UP transformer:| function, look, advantage?

increases voltagemore coils on secondary sideminimizes energy loss

STEP-DOWN transformer:| function, look, advantage?

decreases voltageless coils on secondary sidesafe to use

why does a STEP-UP transformer MINIMIZE ENERGY LOSS?

(constant power, P=IV)| V increase → I decrease → lower I wastes less energy

explain how a TRANSFORMER works

Alternating Vp → Alternating Ip → Alternating MF → Alternating MF induced in iron core → Vs is induced → Is flows

TRANSFORMER:| what material is the core made of? why?

IRONcuz it is SOFT MAGNETIC MATERIAL → can be magnetized (MF can be induced)→ easy to magnetize and demagnetize

TRANSFORMER:| what's the equation relating V and N (number of coil) on primary and secondary sides?

Vp/Np = Vs/Ns

TRANSFORMER:| what's the equation relating V and I on primary and secondary sides?

P = Ip • Vp = Is • Vs

TRANSFORMER:| what type of current is needed at primary side? why?

A.C.| cuz it needs to induce a CHANGING MF